Paul A. Lane

Optical properties of a bio-inspired gradient refractive index polymer lens

The design, fabrication, and properties of one of a new class of gradient-index lenses are reported. The lens is an f/2.25 GRIN singlet based on a nanolayered polymer composite material, designed to correct for spherical aberration. The light gathering and focusing properties of the polymer lens are compared to a homogeneous BK7 glass singlet with a similar f-number. The modulation transfer function of the polymer GRIN lens exceeded that of the homogeneous glass lens at all spatial frequencies and was as much as 3 times better at 5 cyc/mm. The weight of the polymer lens was approximately an order of magnitude less than the homogeneous glass lens.

Efficient, single-layer molecular organic light-emitting diodes

The authors demonstrate efficient molecular organic light-emitting diodes that use direct hole injection from poly(3,4-ethylene-dioxythio-phene):poly(styrene-sulfonate) into a single layer of tris(8-hydroxyquinoline) aluminum (III) for carrier transport and electroluminescence. Single-layer devices have a lower operating bias and higher luminous power efficiency than conventional bilayer devices with a 4,4-bis[N-1-napthyl-N-phenyl-amino]biphenyl hole transport layer. The current density-voltage characteristics of single-layer devices follow Schottky-Richardson behavior and are consistent with an Ohmic contact at the anode.

Influence of carrier injection on the electromodulation response of trap-rich polymer light-emitting diodes

We investigate the influence of carrier injection on the electric field distribution in polyfluorene-based polymer light-emitting diodes containing poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS). The devices show strong charge-induced electromodulation spectra due to the accumulation of trapped electrons close to the PEDOT:PSS/polyfluorene interface. The trapped electrons cause the potential to drop preferentially at the interface, enhancing hole injection and substantially reducing the magnitude of the electric field in the bulk semiconductor. The detailed operating mechanisms of such “trap-rich” devices are poorly understood, and in this paper we perform a series of temperature-dependent current-voltage sweeps and electromodulation measurements to clarify the role of the injected charge. We find that the devices show strong field redistribution only at room temperature and that devices operating at lower temperatures (<100K) resemble trap-free light-emitting diodes with a uniform e...

Electromodulated doping of the hole transport layer in a small molecule organic light-emitting diode

Electromodulation spectroscopy has been used to probe the effect of a polymer hole injection layer on electric fields and charge injection in vacuum-deposited organic light-emitting diodes. The electromodulation spectrum consists of electroabsorption of the transport layers and excited state absorption of trapped cations in the hole transport layer. Field-dependent modulation of trapped charge at the interface between the injection and transport layers substantially modifies the electric field distribution within the device. In reverse bias, the electric field strength is suppressed within the hole transport layer and concentrated in the electron transport layer. In forward bias, field-dependent doping of the hole transport layer dominates the electromodulation spectrum. The field-dependent trap density is calculated to be of order 1013 cm−2, equivalent to μC/cm2 charge density. The built-in potential is estimated to be between 2.2 and 2.5 V, consistent with low carrier injection barriers.

Organic Photovoltaic Cells Using Group 10 Metallophthalocyanine Electron Donors

Organic planar heterojunction solar cells using as an electron acceptor and group 10 metal phthalocyanines (nickel, palladium, and platinum) as electron donors have been fabricated and evaluated for their device characteristics. Upon changing the metal center, these devices exhibit interesting trends, such as variations in the open-circuit voltage, short-circuit current density, and power-conversion efficiency. Devices based on palladium phthalocyanine (PdPc) exhibited the best performance, achieving power-conversion efficiencies of 2.4% under AM1.5G ( ) illumination. This is due to the higher ionization potential of PdPc and greater efficiency of charge photogeneration. Nickel phthalocyanine (NiPc) has the highest hole mobility, but cells using NiPc have the weakest charge-generation efficiency. This is due to poor exciton diffusion to the organic heterojunction that can be overcome by using a composite NiPc: charge-generation layer. An optimized NiPc-based cell has a power-conversion efficiency of 1.9%.

Energy transfer and excitation migration in a doped organic semiconductor

We have studied energy transfer to a dioxolane-substituted pentacene derivative, 6,14-bis-(triisopropylsilylethynyl)-1,3,9,11-tetraoxa-dicyclopenta[b,m]pentacene (TP-5), from tris(8-hydroxyquin-8-olinato) aluminum(III) (Alq3) by steady state and time-resolved photoluminescence (PL) spectroscopy. The Förster transfer radius is 27 Å, calculated from the fluorescence spectrum of Alq3 and the absorption spectrum of TP-5. We find that pentacene emission dominates the PL spectra of TP-5:Alq3 guest-host films, even at concentrations where the typical guest separation is significantly larger than the Förster transfer radius. Monte Carlo simulations of energy transfer to randomly dispersed guest molecules in the host matrix show that Förster-type energy transfer cannot completely account for the PL dynamics of the guest and host. Exciton diffusion within the Alq3 host followed by fluorescence of the host molecules or energy transfer to the guest explains the PL spectra and dynamics.

Reduced photoconductivity observed by time-resolved terahertz spectroscopy in metal nanofilms with and without adhesion layers

Non-contact, optical time-resolved terahertz spectroscopy (TRTS) has been used to study the transient photoconductivity of nanometer-scale metallic films deposited on fused quartz substrates. Samples of 8 nm thick gold or titanium show an instrument-limited (ca. 0.5 ps) decrease in conductivity following photoexcitation due to electron-phonon coupling and subsequent increased lattice temperatures which increases charge carrier scattering. In contrast, for samples of 8 nm gold with a 4 nm adhesion layer of titanium or chromium, a ca. 70 ps rise time for the lattice temperature increase is observed. These results establish the increased transient terahertz transmission sign change of metallic compared to semiconductor materials. The results also suggest nanoscale gold films that utilize an adhesion material do not consist of distinct layers.

Guest Editorial: Special Section on Organic Photovoltaics

Organic photovoltaics (OPVs) have a long history, stretching back three decades into the 1980s, when first studies were conducted on the photogeneration of charge carriers in organic solids. The breakthrough in this field was in 1995, when Heeger’s group published the first efficient solution-processed solar cells based on a bulk heterojunction consisting of a polymer blend with C60, a new concept reported three years earlier by Sariciftci and Heeger. Since then, many active research activities have been undertaken to develop highly efficient organic photovoltaic devices. The number of papers on organic solar cells has been rising exponentially, and the peak of publications has not yet been reached. Tremendous progress has been made in the synthesis and production of organic solar cells. Companies such as Merck, BASF, and Plextronics have started to commercialize organic semiconducting materials, while companies such as Konarka Technologies, Inc. and Heliatek have begun commercialization of organic solar modules. Compared to inorganic solar cells, OPVs offer many advantages, such as low cost, highthroughputproduction,flexibledevices,lightweightproducts,aswellascustom-designedcolors. Onthedownside,OPVsstillhavesignificantlylowerefficiencyvaluesandlifetimeexpectations as compared to their inorganic counterparts. Nevertheless, the most recent National Renewable Energy Lab (NREL) certified power conversion of more than 8%, as reported by Konarka and Heliatek, positions OPVs as the next generation of solar cells and a follow-up technology for thin-film inorganic PVs. In order to achieve higher efficiency and better lifetime, further development is necessary: stable and low-bandgap semiconductors with excellent charge carrier transport properties are required, concepts to control the microstructure in bulk heterojunction composites are essential, the development of efficient and environmentally stable interface materials has to take place, and, finally, strategies for a cost-efficient and long-time stable packaging process need to be developed. In addition, further fundamental understanding of the photophysical processes, including the different interfaces in organic solar cells, is essential to unravel device degradation mechanisms. For the final product release, light propagation and light management need to become integrated in organic solar modules. In this special section of the Journal of Photonics for Energy, papers that address the above issues and challenges are presented. These papers are based on talks and posters given at the conference on Organic Photovoltaics XI at the SPIE Optics + Photonics meeting held in San Diego in August 2010. We believe that readers will find the results of the studies discussed in these manuscripts interesting, educational, and stimulating, and we hope that you will enjoy reading them.

Carrier dynamics of composite and nanolayered films of zinc phthalocyanine and C60 measured by time-resolved terahertz spectroscopy

We present a study of charge transfer and carrier dynamics in films of zinc phthalocyanine (ZnPc) and buckmisnsterfullerene (C60) investigated by time-resolved terahertz spectroscopy (TRTS). These films are model structures for charge generation layers in organic photovoltaics and their intrinsic properties are therefore of interest. We compare two classes of films: composite films of ZnPc and C60 prepared by co-evaporation and layered ZnPc/C60 films prepared by alternating deposition. We find evidence for a short-lived charge transfer state of C60 that decays within several picoseconds of excitation. In contrast, both composite and multi-layered films have a long-lived THz absorption that depends on the composition and structure of the fims. The optimum composition for charge transfer within composite films is a 1:1 blend of ZnPc and C60. Amongst the layered films, there is an increase in charge photogeneration with decreasing layer thickness with a sample having ultrathin (2 nm) exhibiting the strongest THz absorption. A much stronger THz absorption signal was obtained from the layered structure than for the best composite film, even both structures contain similar fractions of ZnPc and C60 .

Electromodulation studies of a polyfluorene light-emitting diode

Electromodulation (EM) spectroscopy has been used to probe the electric field distribution in polymer light-emitting diodes. Below the turn-on bias, the EM spectrum is dominated by electroabsorption of the emissive layer. The electroabsorption signal vanishes at the turn-on bias. Under operation, the EM spectrum is due to by excited state absorption from injected charge and bleaching of the ground state absorption of the emissive layer. We conclude that the internal electric field is effectively screened by accumulation of trapped electrons at the anode.